A great number of improvements in audio equipment, for example, amplifiers or loudspeakers, have been made to produce sounds of higher fidelity. As a result, a recent amplifier or loudspeaker, individually, produces a good performance. However, when the amplifier and loudspeaker are combined, impedance mismatching between the amplifier and loudspeaker becomes a problem. That is, the impedance of the extension wires connecting the amplifier and the loudspeaker together adds to the impedance of the loudspeaker so that an output impedance of the amplifier becomes difficult to match with the whole of the load impedance, including the impedance of the loudspeaker and the extension wires. In such an impedance mismatching condition, an output signal voltage of the amplifier, as well as a counter electromotive force voltage of the loudspeaker, arises across the loudspeaker terminals. The counter electromotive force voltage has nonlinearity characteristics so that the loudspeaker is driven by an output signal voltage which is distorted by the counter electromotive force voltage.
Heretofore, an audio amplifier having a circuit automatically compensating a counter electromotive force of a loudspeaker by means of feedback has been rarely considered. Moreover, it has been found that there is needed a critical characteristic relationship between a line amplifier and a feedback means for compensating.